The present invention relates generally to signal processing systems that process signals non-linearly and, more particularly, to systems that non-linearly process signals in a partially differential configuration.
In a signal processing system it is often necessary to provide a non-linear transfer function to amplify portions of the signal dynamic range by different amounts. This non-linear processing is often necessary for one or more reasons. For example, in a video signal processing system, non-linear processing may be used: (1) to improve the subjective appeal of a displayed video image, (2) to compensate for different xe2x80x9cgammaxe2x80x9d characteristics of cameras and display devices, or (3) to accommodate differing non-linear light output characteristics of the various light output channels of a color video display device.
A common problem in video signal processing systems is pick-up of xe2x80x9cnoisexe2x80x9d signals due to (1) stray electrostatic and/or electromagnetic fields, (2) ground potential differences between stages of the system and (3) supply voltage differences between stages of the system. In a non-linear processing stage, it is also problematic if the bias reference for the signal and the bias reference for the non-linear inflection point thresholds are different, which also adds a form of xe2x80x9cnoisexe2x80x9d to the signal.
In order to minimize stray electrostatic and or electromagnetic pickup in inter-stage connections and/or to minimize ground induced noise due to ground potential differences between circuits in a signal processing system it is often the case that partially differential signaling techniques will be employed, often in connection with twisted pair wiring. In partially differential signaling, one connection will communicate the common-mode bias for the time variant signal communicated on the second connection of the partially differential pair. Subsequent signal processing, such as differential amplifiers, having the capability to reject common-mode variations imposed on the partially differential interconnections, will be employed. It is often the case that portions of the signal processing will be included in an integrated circuit video amplifier (such as the TDA6120Q CRT Driver IC manufactured by Toshiba) wherein the voltage gain is provided by a differential amplifier having an external gain setting emitter degenerating resistor. One example of a particular signal processing system application that may include such an IC and exhibit the described problems is a kinescope driver amplifier in a television system.
The invention resides, in part, in recognizing the problems described above and, in part, in providing a non-linear signal processing system that solves the problem. Such a system comprises a processor for amplifying a partially differential signal, the partially differential signal comprising a time-variant signal and a common-mode bias, and for rejecting a time-variant common-mode signal imposed upon the partially differential signal; means for establishing a threshold level representative of the common-mode bias; means for altering the gain of said processor in response to a predetermined relationship existing between the time-variant signal and the threshold level; and means for substantially preventing the time-variant common-mode signal from modifying the predetermined relationship.